Image projection systems are employed for a number of applications, including large-screen television. Rear projection systems are those in which the image is projected on the rear surface of the screen, the screen is formed of translucent or transparent material, and the image is therefore visible through the screen to observers located on the front side. Such systems are often preferred for projection TV applications, because they permit the projectors and associated optics to be hidden behind the screen. A disadvantage of the rear projection design, however, is that some of the image light is not transmitted forwardly through the screen, but instead is lost due to rearward reflection from the front and rear surfaces of the screen.
More importantly, the fraction of the total image light which is rearwardly reflected is greater at greater distances from the axis of the projection system. Thus, even if the overall brightness is raised to compensate for reflection losses, the brightness will still be non-uniform; it will decrease from the axis to all four edges of the screen. Consequently, at any given viewing location an observer will find the TV picture to be brighter in the center than it is elsewhere. This invention aims to compensate for such non-uniformity.
It is common for projection screens to employ lenticular lenses for optical processing of the image projected thereon. These are parallel arrays of ridges formed in the surface of the screen, which have light-refracting cross-sectional shapes. Each individual ridge, or lenticule, refracts only a small portion of the image; but the array as a whole processes the entire image. In the past such lenses have been formed in rectilinar arrays and used for distributing the image light over a selected range, vertical or horizontal; so that observers at different heights, or at different azimuths relative to the screen, can all view the same projected image. Prior art screens have also used circular or spiral lenticular arrays, called Fresnel lenses, as "field" lenses for collimating the image light, which arrives in the form of divergent rays.
Some prior art projection screens comprise two or more sheets of material, and thus have at least four surfaces (two forwardly facing and two rearwardly facing) on which to place such lenticular arrays. Other prior art screen designs employ only one sheet (or the equivalent, a plurality of sheets bonded face-to-face). Multi-sheet screens are more expensive to manufacture, and they are more prone to reflections which degrade performance. These include rearward reflections which cut down on transmitted light, as well as forward reflections of ambient light which reduce contrast. Single sheet screens are less expensive and less reflective, but the design constraints are more severe because they have only two surfaces on which to place lenticular arrays. Some of the single sheet designs have a Fresnel field lens on one side for collimation, and a vertical rectilinear lenticular array on the other side for horizontal distribution. Since that takes up all the available surfaces, screens of this type often rely upon a light-diffusing (i.e. translucent rather than transparent) layer to spread the image light vertically.
The use of diffusion for this purpose, however, has its advantages. Diffusion by its nature cannot be confined to the vertical direction, and so it affects the distribution of light in the horizontal direction as well. Diffusion is also a less efficient method of image light distribution than refraction, owing to the significant amount of backward reflection, and also some absorption, attributable to the diffusion layer. In addition ambient light is reflected back to the viewer degrading contrast. The amount of diffusion which occurs is also difficult to control.
If the diffusion element is applied as a surface coating, uniform thickness is difficult to achieve. Such coatings also tend to fill in the valleys between the ridges of a lenticular array. If diffusion is achieved by molding the screen with a rough surface texture, then the mold must be carefully maintained to preserve its surface characteristics over a large number of pressings. Furthermore, some of the resin may adhere to the rough surface of the mold, which reduces the ability of the screen to diffuse light, and also makes its removal from the mold more difficult. Another approach to diffusion is the use of light-dispersing pigment granules or other optically active particles mixed with the screen resin before pressing. It is important, when using this approach, to make sure that the diffusion material is distributed uniformly across the screen, or non-uniform brightness distributions may occur, causing poor image quality. On the other hand, if the diffusion material is distributed throughout the thickness of the screen, as is usually the case, the focal plane of the projected image is not distinct, which results in loss of image sharpness. Another problem is that the presence of the diffusion material may affect the molding and handling characteristics of the resin.
The present invention contemplates a single-sheet (or two-surface) screen design which does not employ diffusion for vertical distribution. It employs a fully transparent, nondiffusive material which is formed into a sheet having a vertically running, horizontally dispersing rectilinear lenticular array one one surface, and a horizontally running, vertically dispersing rectilinear lenticular array on the other surface. Since there is no other surface left on which to form a field lens, the collimating function is built into the refractive characteristics of the two lenticular lenses. Since the angle of incidence of the image light varies from 0.degree. at the axis of the projection system to progressively larger angles with increasing distance from the axis, the collimating function requires the refractive properties of the lenticules to vary as a function of screen position. Hence their cross-sectional shape changes from lenticule to lenticule across the surface of the screen.
Change of lenticule shape as a function of screen coordinate was employed by the prior art for this purpose in a number of spiral or circular Fresnel field lens designs and also in some rectilinear lenticular arrays. Some prior art rectilinear lenticular arrays also change the lenticule cross-sectional shape as a function of screen coordinate for another purpose, i.e., in order to compensate for the cosine power fall-off in image light intensity as a function of angle of ray divergence, which results from the inherent characteristics of the projector focussing lenses.
It appears, however, that no prior art screen varies the lenticule cross-sectional shape as a function of screen coordinate in a manner to compensate even partially for the fact that the reflection losses are greater at greater distances from the projection axis. It is especially important to do this in a screen of the present type, because the use of two orthogonal distributive lenticular arrays would otherwise produce unacceptable brightness gradients in both the vertical and horizontal directions due to the reflection differential across the height and width of the screen.
The problem of reflection loss gradient has been recognized in Strong et al., U.S. Pat. No. 2,200,646. That reference, however, is not concerned with one or more angular ranges of viewing positions. It is only concerned with a linear range of viewing positions extending along the axis, with only the perpendicular distance from the screen as a variable parameter. In Strong the projected image is used only as a cinema backdrop, and thus there is only one "observer", i.e., a movie camera which is always set up in some axial position, rather than a plurality of unpredictable human observers who may take viewing positions above, below, or to either side of the axis. Therefore, the brightness gradient is eliminated only for axial viewing positions at various perpendicular distances from the screen. No compensation is provided for a range of off-axis viewing positions dispersed in any direction parallel to the screen, such as horizontally and vertically. If that screen design were used in a projection TV system, all viewers would have to take up single-file positions directly on the axis, otherwise the brightness of the projected image would vary vertically or horizontally or both.
Moreover, the Strong patent uses the same lenticule cross-sectional shape at each location. The limited degree of compensation for differential reflection which is described in that patent results solely from the fact that the light reaching a given axial viewing position, from any two lenticules having different screen coordinates, is refracted from different facets of the lenticule profile. Thus, no use is made of the concept of lenticule shape change for the purpose of compensating reflection differentials.
In the present invention, the lenticules vary as a function of screen coordinate in such manner that reflection differentials are at least partly compensated for each one of a selected range of viewing positions extending in at least one direction parallel to the screen, e.g., vertically and/or horizontally. The variation is such that, for each viewing position within the selected horizontal and/or vertical viewing range, a greater fraction of the incident image light is directed to that viewing position from those lenticules which are further from the axis than is directed to that viewing position from those lenticules which are closer to the axis. This compensates at least partially for the greater image light loss due to rearward reflection suffered by those lenticules which are further from the axis, as compared to those lenticules which are closer to the axis.
In the preferred form of the invention, both the vertical and horizontal lenticular arrays are so compensated. Thus, at any viewer position within a selected horizontal viewing angle and a selected vertical viewing angle, the observer will see a projected image field which is uniform in brightness from top to bottom and from left to right.
The foregoing background discussion and brief description of the invention, as well as its features and advantages, will be more fully understood by reference to the following detailed description of the preferred embodiment of the invention, when read in conjunction with the following drawings.